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A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin, is a two-cylinder
piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common feat ...
whose cylinders are arranged in a line along a common crankshaft. Straight-twin engines are primarily used in motorcycles; other uses include automobiles, marine vessels, snowmobiles, Jet Skis, all-terrain vehicles, tractors and ultralight aircraft. Various different crankshaft configurations have been used for straight-twin engines, with the most common being 360 degrees, 180 degrees and 270 degrees.


Terminology

The straight-twin layout is also referred to as "parallel-twin", "vertical-twin" and "inline-twin". Some of these terms originally had specific meanings relating to the crankshaft angle or engine orientation, however they are often also used interchangeably. In the United Kingdom, the term "parallel-twin" is traditionally used for engines with a crankshaft angle of 360 degrees, since the two pistons are in the same direction (i.e. parallel to each other). "Vertical-twin" was used to describe engines with a crankshaft angle of 180 degrees, which causes the pistons to travel in opposite directions. The terms "straight-twin" and "inline-twin" were used more generically for any crankshaft angle. For motorcycles, "inline-twin" has sometimes referred to either a
longitudinal engine In automotive engineering, a longitudinal engine is an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, front to back. Use This type of motor is usually used for rear-wheel drive cars, except f ...
orientation (i.e. with the crankshaft in line with the chassis) or a U-engine ( tandem twin) where the cylinders are arranged longitudinally in the chassis (although the two crankshafts are actually oriented transversely).


Design

Compared with V-twin engines and flat-twin engines, straight-twins are more compact, a simpler design and cheaper to produce. Straight-twin engines can be prone to vibration, either because of the irregular firing interval present in 180° crank engines or the large uncountered reciprocating mass in 360° crank engines. Inline-twins also suffer further from torsional torque reactions and vibration.


Crankshaft angle

The most common
crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecti ...
configurations for straight-twin engines are 360 degrees, 180 degrees and 270 degrees. ; 360 degrees In an engine with a 360 degree crankshaft, both pistons move up and down at the same time. However the firing interval is offset between cylinders, with one of the cylinders firing during the first crankshaft rotation and then the other cylinder in the following rotation. The 360 degree engines can use a single ignition system for both cylinders, using a wasted spark system. The imperfect primary balance is as per a single-cylinder engine of equivalent reciprocating mass. Early engines attempted to reduce vibration through counterweights on the crankshaft, however later methods also included
balance shaft Balance shafts are used in piston engines to reduce vibration by cancelling out unbalanced dynamic forces. The counter balance shafts have eccentric weights and rotate in opposite direction to each other, which generates a net vertical force. T ...
s and a separate weighted connecting rod. Compared with a single-cylinder engine, the more frequent firing interval (360 degrees compared with 720 degrees) results in smoother running characteristics, despite the similar dynamic imbalance. From the 1930s, most British four-stroke straight-twin motorcycle engines used a 360 degree crankshaft, since this avoided the uneven intake pulsing of other configurations, thus preventing the need for twin carburettors. In the 1960s, even though Japanese motorcycles mostly switched to 180 degree crankshafts for engines sized from 250 to 500 cc, various smaller and larger engines continued to use a 360 degree crankshaft. Vibration was less of an issue for smaller engines, such as the 1965 Honda CB92 and 1979 Honda CM185. Larger engines, such as the 1969
Yamaha XS 650 The Yamaha XS650 is a mid-size motorcycle made by Yamaha Motor Company. The standard model was introduced in October 1969,and produced through 1979. The "Special" cruiser model was introduced in 1978 and produced through 1985. The XS650 began with ...
and 1972 Yamaha TX750, often used balance shafts to reduce the vibration. The later 1978-1984
Honda CB250N/CB400N The Honda CB250N and CB400N Super Dream are motorcycles manufactured by the Honda Motor Company from 1978 to 1986. The successor to the short lived Dream model, it had a series of revisions including a six-speed transmission and what Honda termed ...
engines also used a 360 degree crankshaft. The 2008 BMW F series parallel-twin motorcycles also use 360 degree crankshafts, with a third "vestigial" connecting rod (acting as a counterbalance) and a rev limit of 9,000 rpm to reduce vibrations. ; 180 degrees In an engine with a 180 degree crankshaft, one piston rises as the other falls. In a four-stroke engine, the firing interval is uneven, with the second cylinder firing 180 degrees after the first, followed by a gap of 540 degrees until the first cylinder fires again. The uneven firing interval causes vibrations and results in a 'lumpy' power delivery. A 180° engine also requires a separate ignition system for each cylinder. Perfect primary balance is possible with a 180 degree straight-twin engine, however the design creates a rocking couple which requires use of a balance shaft to reduce the vibration. A 180 degree straight-twin engine has a secondary imbalance (similar to an inline-four engine), however the lower reciprocating mass means that this often does not require treatment. A 180° crankshaft engine suffers fewer pumping losses than a 360° twin, because displacement of the crankcase is relatively unchanged as the pistons move. In the 1960s, Japanese motorcycle manufacturers favoured the use of 180 degree crankshafts, since the increased smoothness allowed higher rpm and thus higher power outputs. For example, the 1966 Honda CB450 180 degree crankshaft engine has a similar power output to contemporary British 360 degree crankshaft engines, despite having a smaller displacement of 450 cc compared with 650 cc. Both the 1973 Yamaha TX500 and the 1977 Suzuki GS400 had a 180 degree crankshaft and a balance shaft. Since 1993, most Honda straight-twin motorcycle engines use 180 degree crankshafts.
Two-stroke engine A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of ...
s typically use a 180 degree crankshaft, since this results in two evenly-spaced power strokes per revolution. The
fundamental frequency The fundamental frequency, often referred to simply as the ''fundamental'', is defined as the lowest frequency of a periodic waveform. In music, the fundamental is the musical pitch of a note that is perceived as the lowest partial present. I ...
of vibration is twice that of an equivalent single-cylinder engine, however the amplitude is halved. Two-stroke engines that do not use a 180 degree crankshaft include the 1972
Yankee The term ''Yankee'' and its contracted form ''Yank'' have several interrelated meanings, all referring to people from the United States. Its various senses depend on the context, and may refer to New Englanders, residents of the Northern United S ...
. ; 270 degrees In an engine with a 270 degree crankshaft, one piston follows three quarters of a rotation behind the other. This results in an uneven firing interval where the second cylinder fires 270 degrees after the first, followed by a gap of 450 degrees until the first cylinder fires again. This is the same pattern as a 90 degree V-twin engine, and both configurations have a similar 'pulsing' exhaust sound as a result. The pistons in a 270 degree straight-twin engine are never both stationary at the same time (as per a 90 degree V-twin engine), thereby reducing the net momentum exchange between the crank and pistons during a full rotation. An imperfect primary balance is created in a 270 degree straight-twin engine, due to a combination of free force and rocking couple; a balance shaft is often used to compensate for this. The secondary balance of a 270 degree engine is perfect, however the configuration does result in an unbalanced rocking couple. The first production 270 degree straight-twin motorcycle engines were fitted to the 1996 Yamaha TRX850 and Yamaha TDM. Later examples include the 2009 Triumph Thunderbird, 2010 Norton Commando 961, 2012
Honda NC700 series The Honda NC700 series is a family of motorcycles produced by Honda since 2012. NC700 series was a 'new concept', being unlike conventional motorcycles, a bike designed for commuters, new or veteran riders. The series also includes the motorcycle/ ...
, 2014 Yamaha MT-07, 2016 Triumph Thruxton 1200, and 2018 Royal Enfield Interceptor 650 & continental gt ,.


Main bearings

Each cylinder in a straight-twin engine has a separate
crank pin A crankpin or crank pin, also known as a rod bearing journal, is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to th ...
, unlike V-twin engines which can use a common crank pin for both
connecting rod A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crank ...
s. Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two main bearings. Beginning in the late 1950s, most Honda straight-twin engines had four main bearings. Subsequent straight-twin engines had four or occasionally three main bearings.


Usage in motorcycles


History

The world's first production motorcycle, the 1894
Hildebrand & Wolfmüller The Hildebrand & Wolfmüller was the world's first production motorcycle. Heinrich and Wilhelm Hildebrand were steam-engine engineers before they teamed up with Alois Wolfmüller to produce their internal combustion ''Motorrad'' in Munich in 18 ...
used a straight-twin engine. The cylinders lay flat and forward-facing, with the pistons connected directly to the rear wheel using a locomotive-style connecting rod. In 1903, the Werner Motocyclette became the second production motorcycle model, using a straight-twin engine with vertical cylinders. The Werner engine uses cast-iron cylinders with integral heads, side valves and has a displacement of 500 cc. The 1938 Triumph Speed Twin was a successful straight-twin motorcycle which also led to straight-twin engines becoming more widely used by other brands. The engine was designed by Edward Turner and
Val Page Valentine Page (1891–1978)Triumph 6/1 sidecar hauler (which won the International Six Days Trial silver medal and the 1933 Maudes Trophy). During the development of the engine, it was found that a 360 degree crank angle was better suited to the use of a single carburettor than a 180 degree crank angle. Following the trend created by the Triumph Speed Twin, the most common design used by British motorcycle manufacturers until the mid-1970s was a four-stroke straight-twin engine with a 360 degree crankshaft. The manufacturers producing these motorcycles included BSA, Norton, Triumph, Ariel, Matchless and
AJS A. J. Stevens & Co. Ltd was a British automobile and motorcycle manufacturer in operation from 1909 to 1931. The company was founded by Joe Stevens in Wolverhampton, England. After the firm was sold, the name continued to be used by Matchless, ...
. Straight-twin engines were also produced by Italian and German manufacturers, along with the American manufacturer
Indian Indian or Indians may refer to: Peoples South Asia * Indian people, people of Indian nationality, or people who have an Indian ancestor ** Non-resident Indian, a citizen of India who has temporarily emigrated to another country * South Asia ...
. In 1949, the AJS E-90 Porcupine won the 500  1949 Grand Prix World Championship, becoming the first and only straight-twin motorcycle to win the championship. This engine is one of few four-stroke straight-twins to use cylinders oriented horizontally rather than vertically. Since the 2000s, BMW and several Japanese manufacturers have continued to produce straight-twin engines, mostly for middleweight models. Several large
scooters Scooter may refer to: Vehicles Ground Human or gravity powered * Eccentric-hub scooter, propelled by a standing rider making a bouncing motion * Kick scooter, propelled by a standing rider pushing off the ground * Knee scooter, a mobility dev ...
have also used straight-twin engines, such as the 2001
Yamaha TMAX The Yamaha TMAX (or T-Max) series of maxi-scooters has been manufactured by Yamaha Motor Company for the European market since its debut at July 2000 press events in Naples, Italy and Iwata, Japan, combining motorcycle performance with the co ...
and the 2001 Honda Silver Wing. Straight-twin engines are also used in motorcross sidecar racing.


Transverse-engined models

Many large British motorcycles from 1945 to the 1960s used a straight-twin
transverse engine A transverse engine is an engine mounted in a vehicle so that the engine's crankshaft axis is perpendicular to the direction of travel. Many modern front-wheel drive vehicles use this engine mounting configuration. Most rear-wheel drive vehicle ...
(i.e. oriented with the crankshaft perpendicular to the frame), such as the Triumph Bonneville and Norton Commando. This layout is well suited to air-cooling, since both cylinders receive equal airflow and the exhaust can exit in the well-cooled location at the front of each cylinder. The transverse-engine straight-twin design has been largely replaced by V-twin engines, however the straight-twin design has the advantage of easier packaging of ancillaries (such as the air-filter, carburettor and ignition components), which also improves access to ancillaries for maintenance/repairs. A straight-twin engine using a 270 degree crankshaft can have a similar sound and feel to a V-twin engine with an uneven firing order.


Longitudinal-engined models

Longitudinal engine In automotive engineering, a longitudinal engine is an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, front to back. Use This type of motor is usually used for rear-wheel drive cars, except f ...
straight-twin motorcycles are less common, however examples include the 1930-1938 Dresch Monobloc and the 1949-1956
Sunbeam S7 and S8 The Sunbeam S7 and S8 are British motorcycles designed by Erling Poppe with styling loosely based on the BMW R75 designs that were acquired as war reparations by BSA (full rights to the Sunbeam brand had been acquired from AMC in 1943). B ...
. This engine orientation allows for a motorcycle as narrow as a single-cylinder engine, which reduces the aerodynamic drag, especially for the purpose of motorcycle racing. However, the main disadvantage for air-cooled engines is that the rear cylinder runs hotter than the front cylinder.


Usage in automobiles

Although two-cylinder engines are quite uncommon in cars, the straight-twin layout has been used for several automobile engines over time. The first known straight-twin engine was a variant of the Daimler Motors' ''Phoenix'' engine introduced in 1895; these engines were used in
Panhard Panhard was a French motor vehicle manufacturer that began as one of the first makers of automobiles. It was a manufacturer of light tactical and military vehicles. Its final incarnation, now owned by Renault Trucks Defense, was formed ...
cars that year. Another early straight-twin engined car was the 1898 Decauville Voiturelle, which used a pair of cylinders taken from a de Dion model mounted fore and aft and positioned below the seat. Straight-twin engines have been used in various small cars, such as the 1957 Fiat 500, 1958 Subaru 360, 1958 NSU Prinz, 1962 Mitsubishi Minica, 1967 Honda N360, 1970
Honda Z600 The Honda Z (marketed also as the Z600) is a two-door hatchback kei car/city car manufactured and marketed by the Honda Motor Company, from 1970 until 1974. Exports mostly ended after 1972, when the domestic market models received redesigned pi ...
, 1972 Fiat 126, 1988
VAZ Oka The Lada Оkа (VAZ-1111, SeAZ-1111, КаmАZ-1111, Astro 11301) (russian: Ока (ВАЗ-1111, СеАЗ-1111, КамАЗ-1111)) is a city car designed in the Soviet Union in the later part of the seventies by AvtoVAZ. It entered production in 19 ...
, 1988 Dacia Lăstun, and 1980 Daihatsu Cuore. Petrol straight-twin engines currently used in production cars include the Tata Nano and the Fiat TwinAir engine (used in various models from Fiat, Lancia and Alfa Romeo). Diesel straight-twin engines currently in production include the Piaggio Porter engine.


Usage in marine vessels

Straight-twin engines have been often used as inboard motors, outboard motors and jet pump motors. In the early 20th century, gaff-rigged British fishing boats such as Morecambe Bay Prawners Lancashire Nobbys would sometimes retrofit an inboard engine, such as the Lister or the ''
Kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...
E2'' 3.0 litre petrol-paraffin engine. From the 1950s, manufacturers of outboard motors had settled on the use of the basic inline engine design, cylinders stacked on top of each other with the crankshaft driving the propeller shaft. The ''Suzuki 15'' outbound motor was introduced in 1989.


Other uses

Other uses include
tractor A tractor is an engineering vehicle specifically designed to deliver a high tractive effort (or torque) at slow speeds, for the purposes of hauling a trailer or machinery such as that used in agriculture, mining or construction. Most commo ...
s (such as various John Deere models until 1960), snowmobiles,
personal watercraft A personal watercraft (PWC), also called water scooter or jet ski, is a recreational watercraft that a rider sits or stands on, not within, as in a boat. PWCs have two style categories, first and most popular being a runabout or "sit down" whe ...
s, and
all-terrain vehicle An all-terrain vehicle (ATV), also known as a light utility vehicle (LUV), a quad bike, or simply a quad, as defined by the American National Standards Institute (ANSI); is a vehicle that travels on low-pressure tires, with a seat that is stra ...
s. Design variations include two-stroke, four-stroke, petrol, diesel, air-cooling, water-cooling, natural aspiration and turbocharging.
Ultralight aircraft Ultralight aviation (called microlight aviation in some countries) is the flying of lightweight, 1- or 2-seat fixed-wing aircraft. Some countries differentiate between weight-shift control and Aircraft flight control system, conventional three-a ...
, single seat gyro-copters and small homebuilt aircraft have also used straight-twin engines, often using engines originally designed for snowmobiles such as the
Hirth 2704 The Hirth 2704 and 2706 are a family of in-line twin cylinder, two stroke, carburetted aircraft engines, with optional fuel injection, designed for use on ultralight aircraft and especially two seat ultralight trainers, single seat gyrocopte ...
and Cuyuna 430-D. Purpose-built engines for ultralight aircraft include the Rotax 503 and Rotax 582. Straight-twin engines are sometimes also used in large scale radio-controlled aircraft.


See also

* Flat-twin engine * List of motorcycles by type of engine * V-twin engine


References

{{Piston engine configurations 2